Poly Tanks & Plastic Water Storage Tanks: Sizing and Selection Guide
How to size and specify a storage poly tank: capacity and footprint chart, specific gravity ratings, chemical compatibility, and why a water tank is not a chemical tank.
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TL;DR: A storage poly tank is a one-piece polyethylene vessel for water and chemical storage, rated by capacity and by the maximum specific gravity of the liquid it can hold. Sizing requires three numbers, not one: gallons, footprint and specific gravity. A 1.5 SG rated tank holds a 1.9 SG acid at only about 79 percent of its stated volume, and a potable water tank needs FDA-compliant resin.
Quick Facts
| Spec | Typical range |
|---|---|
| Capacity | 65 to 20,000+ gallons |
| Material | Linear or cross-linked polyethylene (HDPE / XLPE) |
| Specific gravity rating | 1.5 SG standard, 1.9 SG heavy-duty |
| Service temperature | 32 F to about 120 F continuous |
| Wall construction | One-piece, seamless, no welds |
| Potable water | Requires FDA 21 CFR 177.1520 compliant resin; NSF/ANSI 61 where required |
| Service life | Commonly 10 to 20 years indoors, shorter in high-UV outdoor use |
| Fittings | Bulkhead, threaded outlet, vented manway |
Sizing: The Three Numbers That Matter
1. Capacity. Size to peak demand plus reserve, not average demand. For process water, size to the largest single batch plus refill lead time. Verify usable volume against the tank's outlet height — the volume below a side-mounted outlet is dead storage you cannot draw.
2. Footprint and height. Tall narrow tanks save floor space and raise the center of gravity, which increases anchoring requirements. Short wide tanks are more stable but consume floor area. Confirm door and ceiling clearance for the assembled tank, not the shipping dimension.
3. Specific gravity. This is the number most often skipped and the one that fails tanks.
Effective capacity = rated capacity x (tank SG rating / product SG)
A 1,000 gallon tank rated at 1.5 SG storing a 1.9 SG sulfuric acid solution can only be filled to roughly 790 gallons. Filling it to the gallon marking overloads the wall. When in doubt, specify the higher SG rating; the cost delta is far smaller than a failure.
| Product | Typical SG | Minimum tank rating |
|---|---|---|
| Water, most food liquids | 1.0 | 1.5 |
| Brine, syrups | 1.2 to 1.4 | 1.5 |
| Sodium hydroxide 50% | 1.53 | 1.9 |
| Sulfuric acid 93% | 1.83 | 1.9 |
| Ferric chloride | 1.4 to 1.5 | 1.9 |
| Diesel, oils, most solvents | 0.8 to 0.95 | 1.5 |
HDPE vs Fiberglass vs Steel
| Factor | HDPE / poly | Fiberglass (FRP) | Steel |
|---|---|---|---|
| Corrosion | Immune to most acids, bases, salts | Resin-dependent, liner-dependent | Corrodes; needs coating or lining |
| Weight | Lightest — often one-person handling at small sizes | Moderate | Heaviest |
| Seams | None — one-piece molded | Bonded seams | Welded seams |
| Installed cost | Lowest at small and mid capacity | Higher | Highest at small capacity |
| Repairability | Limited | Patchable | Weldable |
| UV | Requires UV-stabilized resin | Gel coat degrades | Coating degrades |
| Failure mode | Gradual stress crazing | Delamination | Pitting, through-corrosion |
Below roughly 10,000 gallons and in corrosive service, polyethylene is normally the lowest total cost. Above that, or where high temperature and pressure are involved, FRP and steel start to win. Our HDPE vs fiberglass vs steel tank comparison works through the crossover in detail.
A Water Tank Is Not a Chemical Tank
Three differences make this non-interchangeable:
Never repurpose a used tank into a different chemical service without documented knowledge of the prior contents and a compatibility check on every wetted component.
Potable Water and Food Contact
For drinking water, process water in a food plant, or any liquid that contacts food:
See FDA-compliant plastic tanks for what we quote, and the FDA food-contact materials guide for the documentation an auditor asks to see.
Outdoor Installation Checklist
Run the numbers before ordering with our tank volume calculator.
> Have a liquid, a volume and a footprint? Send them over and we will quote in 24 hours. Ships nationwide and to Mexico.
FAQ
What is a poly tank?
A one-piece polyethylene vessel used for water and chemical storage. Because it is molded seamlessly rather than welded or bonded, it has no seams to leak and is immune to corrosion from most acids, bases and salts.
What size is a 275 gallon water tank?
Capacity is standardized but footprint is not — vertical, horizontal transport and cone-bottom designs at 275 gallons all occupy different space. Specify footprint and height alongside gallons, and check the outlet height for dead storage.
What specific gravity rating do I need?
At least the specific gravity of the stored liquid. Water and most food liquids fit a 1.5 SG tank; concentrated caustics and acids at 1.5 to 1.9 SG require a 1.9 SG rated tank.
Can I store chemicals in a water tank?
Not without verifying the specific gravity rating, resin compatibility, gasket material and venting. Each of those can differ from a potable water specification, and any one of them can cause a failure.
Are plastic water tanks safe for drinking water?
Yes, when the resin is FDA 21 CFR 177.1520 compliant and, where required, NSF/ANSI 61 certified, and the tank is fitted with a screened vent and a drainable outlet for sanitation.
How long does a poly tank last?
Commonly 10 to 20 years indoors. Outdoor life depends on UV exposure and resin stabilization; high-UV regions shorten it, and dark UV-stabilized pigments extend it.
Sources
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